In the manufacture of lamps, i.e., fluorescent, high intensity discharge lamps, a phosphor layer is typically coated on the interior surface of a glass envelope using an aqueous suspension of phosphor powder. The phosphor coating on the interior surface of the glass envelope desirably converts ultraviolet radiation emitted by the mercury discharge to emitted light of the correct color, typically visible illumination having a white color.
Aqueous phosphor coating suspensions are described in many U.S. patents. U.S. Pat. No. 3,833,398 to Schreurs describes a process for improving the adhesion of a phosphor suspension. Calcium halophosphate activated with antimony and manganese is mixed with sub-micron size aluminum oxide particles and an ammonium lignosulfonate solution and the resulting solids separated. The resulting solids are then mixed with an aqueous solution containing polyoxyethylene to form a phosphor coating suspension.
Other patents relating to aqueous phosphor suspensions include the following. U.S. Pat. No. 3,999,993 to Patel et al describes the controlled addition of ammonium hydroxide to an aqueous rare earth phosphor slurry to prevent the formation of rare earth chromate complexes. U.S. Pat. No. 4,148,935 to Schreurs describes a two coat process for a fluorescent lamp. The first coating solution of aluminum oxide contains polyoxyethylene, hydroxyethylmethyl cellulose, and glyoxal. In U.S. Pat. No. 4,340,512 to Schreurs, a temporary organic binder comprises purified carboxy methyl cellulose and polyethylene oxide.
U.S. Pat. No. 5,000,989 to Ford describes the preparation of a fine particle-size aqueous silica coating suspension including an aqueous base such ammonium hydroxide, a defoaming agent, a surface active agent, an insolubilizing agent, and a plasticizer. A pair of water soluble binders, preferably hydroxyethylcellulose and poly (ethylene oxide) are added so that a uniform silica coating results. The above patent is discussed in U.S. Pat. No. 5,051,653 to DeBoer where the above described suspension is utilized to deposit a reflecting layer prior to coating a phosphor layer.
In addition to correct color with good lumen output and adaptability to the coating and baking process of lamp manufacture, the phosphors used in lamps must maintain their light output for thousands of hours. The term "lumen maintenance" is generally used in discussions of this changing light output with time and it is sometimes abbreviated to "maintenance".
Many factors affect lumen maintenance. Glass composition, pressure of gas fill in the lamp, choice of the rare gas, wattage per square inch of phosphor area, chemical additions to the coating suspension, lamp processing procedure, phosphor particle size, and surface treatment are all involved.
When a lamp which has operated for a considerable length of time is opened, the surface layer of the phosphor which is exposed to the arc is likely to be distinctly gray. It is speculated that possible causes of the grayness are absorption of a layer of finely divided metallic mercury, deposition of mercurous oxide, or decomposition of the phosphor with liberation of free metals. Other possible causes are decomposition reactions involving activator centers with migration of the metal to the surface and the forming color centers of various types. Since graying of the phosphor also occurs in high-pressure mercury vapor lamps where phosphor is not exposed to bombardment by mercury ions, it is likely that the graying may be caused by a photochemical reaction. Residual hydrocarbon resulting from incomplete combustion of organic binder can create phosphor discoloration leading to brightness loss. The decomposition of hydrocarbon can lead to further maintenance loss.
In the preparation of lamps from a coating suspension containing a chlorofluorophosphate activated with antimony and manganese, in particular a phosphor having the formula Ca.sub.5 -x-ySb.sub.x Mn.sub.y (PO.sub.4).sub.3 Cl.sub.z F.sub.1-x-z O.sub.x, there is a loss in weight percentage of halogen in the phosphor. This loss of halogen from the phosphor creates crystal defects that can lead to the formation of color centers and the deterioration of the phosphor resulting in poor lumen maintenance of phosphor. Hence, it desirable to reduce or eliminate hydrocarbons and the loss of halogen from the phosphor during lamp manufacture.